As a method for producing a single crystal, the CZ method by which a crystal is pulled upwardly from raw material melt in a crucible while being grown has been widely practiced. In producing a silicon single crystal by the CZ method, by adjusting the rate of flow of argon, the furnace pressure, and the distance (hereinafter also referred to as a melt surface space) between a melt surface and a lower end part of an in-furnace structure, control of the concentration of oxygen in the crystal is performed and the number of crystal defects such as OSFs is reduced.
For example, in Patent Document 1, a method for adjusting the melt surface to a predetermined position with reference to the height position of a seed crystal by adding a sensor to a pulling apparatus is proposed. Moreover, in Patent Document 2, a method for adjusting the melt surface space by projecting light onto the melt surface and detecting the light reflected from the melt surface is proposed.
Furthermore, in Patent Document 3, a method for adjusting the melt surface space by bringing a position measuring apparatus into direct contact with melt is described. In addition, in Patent Document 4, a method for adjusting the melt surface space by detecting the projected image of a lower part of a radiation prevention cylinder and moving the radiation prevention cylinder upward or downward is proposed. Moreover, in Patent Document 5, a method for measuring the relative distance between a reference reflector disposed above the melt surface and the melt surface is proposed.